System and method for mr imaging in inhomogeneous magnetic fields

ABSTRACT

An apparatus and method for MR imaging in inhomogeneous magnetic fields includes acquisition of a plurality of three-dimensional (3D) MR data sets, each data set having a central transmit frequency and a central receive frequency set to a frequency offset that is distinct for each 3D MR data set. A composite image is generated based on the plurality of 3D MR data sets.

BACKGROUND OF THE INVENTION

The present invention relates generally to magnetic resonance (MR)imaging and, more particularly, MR imaging in inhomogeneous magneticfields.

When a substance such as human tissue is subjected to a uniform magneticfield (polarizing field B₀), the individual magnetic moments of thespins in the tissue attempt to align with this polarizing field, butprecess about it in random order at their characteristic Larmorfrequency. If the substance, or tissue, is subjected to a magnetic field(excitation field B₁) which is in the x-y plane and which is near theLarmor frequency, the net aligned moment, or “longitudinalmagnetization”, M_(Z), may be rotated, or “tipped”, into the x-y planeto produce a net transverse magnetic moment M_(t). A signal is emittedby the excited spins after the excitation signal B₁ is terminated andthis signal may be received and processed to form an image.

When utilizing these signals to produce images, magnetic field gradients(G_(x), G_(y), and G_(z)) are employed. Typically, the region to beimaged is scanned by a sequence of measurement cycles in which thesegradients vary according to the particular localization method beingused. The resulting set of received NMR signals are digitized andprocessed to reconstruct the image using one of many well knownreconstruction techniques.

The use of MR in musculoskeletal (MSK) diagnostics is a rapidly growingfield. Arthroplasty is the surgical placement of implants. Thepopulation of patients having some form of metal implant is quite largeand growing rapidly. MR has significant capabilities in assisting thediagnosis of implant revisions. Using magnetic resonance imaging toassist in clinical diagnostics of MR-compatible arthroplastic implants,however, has proven a fundamentally challenging problem. Most materialsthat are robust and durable enough to utilize for bone replacements willhave magnetic properties that, when placed in a typical B₀ magneticfield, induce extraneous fields of amplitude and spatial variation thatare large compared to the field offsets utilized in conventional spatialencoding. Accordingly, these materials can introduce distortions in themain magnetic field resulting in an inhomogeneous magnetic field.

While the signal loss induced by these field gradients can largely beregained through the use of Hahn spin-echoes, the distortion theyproduce in both the readout and slice directions are drastic and aretypically unacceptable for clinical evaluation. Despite thesechallenges, MRI has been shown to be quite useful in the diagnosis ofdegenerative conditions in arthroscopic patients. In particular, MRI hasbeen used to screen perioprosthetic soft tissues, diagnose osteolysis,and visualize implant interfaces. These diagnostic mechanisms benefitsignificantly from visual information near implant interfaces.Unfortunately, artifacts induced by the implants in conventional MRIimages are most severe near the implant interfaces.

A proposed approach to reducing MRI artifacts induced by implants is 2DFSE imaging using View-Angle Tilting (VAT). Though this approach canimprove in-plane distortions at the cost of significant image blurring,it does not address distortions in the slice-selection direction. Nearthe most paramagnetic of utilized metallic implants, distortions in theslice-selection direction can almost completely disfigure 2D MR images.While a slice-distortion correction of VAT images in the slice directionhas been proposed, its use is limited because it does not correctsignal-pileup effects of image distortion.

It would therefore be desirable to have a system and method capable ofreducing image artifacts near or around implant interfaces. It wouldfurther be desirable to improve clinical diagnostic access to regions ofinterest near or around implant interfaces.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with an aspect of the invention, a magnetic resonanceimaging (MRI) apparatus includes an MRI system having a plurality ofgradient coils positioned about a bore of a magnet and an RF transceiversystem and an RF switch controlled by a pulse module to transmit RFsignals to an RF coil assembly to acquire MR images. The MRI apparatusalso includes a computer programmed to acquire a plurality ofthree-dimensional (3D) MR data sets, each 3D MR data set acquired usinga central transmit frequency and a central receive frequency set to anoffset frequency value that is distinct for each 3D MR data set. Thecomputer is also programmed to generate a composite image based on theplurality of 3D MR data sets.

In accordance with another aspect of the invention, a method of magneticresonance (MR) imaging includes determining a distinct central frequencyfor each of a plurality of 3D MR data acquisitions and performing theplurality of 3D MR data acquisitions, each 3D MR data acquisition havinga central transmit frequency and a central receive frequency set to thedistinct central frequency determined therefor. The method also includesgenerating a composite image from the plurality of 3D MR dataacquisitions.

In accordance with yet another aspect of the invention, a computerreadable storage medium having stored thereon a computer programincludes instructions which when executed by a computer cause thecomputer to set a center transmission frequency and a center receptionfrequency of a first 3D MR acquisition equal to a first center frequencyoffset and to execute the first 3D MR acquisition to acquire a first setof 3D MR data. The instructions also cause the computer to set a centertransmission frequency and a center reception frequency of a second 3DMR acquisition equal to a second center frequency offset different thanthe first center frequency offset and to execute the second 3D MRacquisition to acquire a second set of 3D MR data. The instructionsfurther cause the computer to reconstruct a composite image based on thefirst and second sets of 3D MR data.

Various other features and advantages will be made apparent from thefollowing detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate embodiments presently contemplated for carryingout the invention.

In the drawings:

FIG. 1 is a schematic block diagram of an exemplary MR imaging systemincorporating embodiments of the present invention.

FIG. 2 is a flowchart of an MR imaging technique according to anembodiment of the invention.

FIG. 3 is a technique for constructing a magnetic field map according toan embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An apparatus and method is provided that acquires multiple 3D MR datasets where the center transmission frequency and the center receptionfrequency of each 3D MR data acquisition are set to an offset frequencythat is distinct for each 3D MR data set. A single image is constructedfrom the 3D MR data sets having reduced artifacts and reduced imagedistortion.

Referring to FIG. 1, the major components of an exemplary magneticresonance imaging (MRI) system 10 incorporating embodiments of thepresent invention are shown. The operation of the system is controlledfrom an operator console 12 which includes a keyboard or other inputdevice 13, a control panel 14, and a display screen 16. The console 12communicates through a link 18 with a separate computer system 20 thatenables an operator to control the production and display of images onthe display screen 16. The computer system 20 includes a number ofmodules which communicate with each other through a backplane 20 a.These include an image processor module 22, a CPU module 24 and a memorymodule 26 that may include a frame buffer for storing image data arrays.The computer system 20 is linked to archival media devices, permanent orback-up memory storage or a network for storage of image data andprograms, and communicates with a separate system control 32 through ahigh speed serial link 34. The input device 13 can include a mouse,joystick, keyboard, track ball, touch activated screen, light wand,voice control, or any similar or equivalent input device, and may beused for interactive geometry prescription.

The system control 32 includes a set of modules connected together by abackplane 32 a. These include a CPU module 36 and a pulse generatormodule 38 which connects to the operator console 12 through a seriallink 40. It is through link 40 that the system control 32 receivescommands from the operator to indicate the scan sequence that is to beperformed. The pulse generator module 38 operates the system componentsto carry out the desired scan sequence and produces data which indicatesthe timing, strength and shape of the RF pulses produced, and the timingand length of the data acquisition window. The pulse generator module 38connects to a set of gradient amplifiers 42, to indicate the timing andshape of the gradient pulses that are produced during the scan. Thepulse generator module 38 can also receive patient data from aphysiological acquisition controller 44 that receives signals from anumber of different sensors connected to the patient, such as ECGsignals from electrodes attached to the patient. And finally, the pulsegenerator module 38 connects to a scan room interface circuit 46 whichreceives signals from various sensors associated with the condition ofthe patient and the magnet system. It is also through the scan roominterface circuit 46 that a patient positioning system 48 receivescommands to move the patient to the desired position for the scan.

The gradient waveforms produced by the pulse generator module 38 areapplied to the gradient amplifier system 42 having Gx, Gy, and Gzamplifiers. Each gradient amplifier excites a corresponding physicalgradient coil in a gradient coil assembly generally designated 50 toproduce the magnetic field gradients used for spatially encodingacquired signals. The gradient coil assembly 50 forms part of a magnetassembly 52 which includes a polarizing magnet 54 and a whole-body RFcoil 56. In an embodiment of the invention, RF coil 56 is amulti-channel coil. A transceiver module 58 in the system control 32produces pulses which are amplified by an RF amplifier 60 and coupled tothe RF coil 56 by a transmit/receive switch 62. The resulting signalsemitted by the excited nuclei in the patient may be sensed by the sameRF coil 56 and coupled through the transmit/receive switch 62 to apreamplifier 64. The amplified MR signals are demodulated, filtered, anddigitized in the receiver section of the transceiver 58. Thetransmit/receive switch 62 is controlled by a signal from the pulsegenerator module 38 to electrically connect the RF amplifier 60 to thecoil 56 during the transmit mode and to connect the preamplifier 64 tothe coil 56 during the receive mode. The transmit/receive switch 62 canalso enable a separate RF coil (for example, a surface coil) to be usedin either the transmit or receive mode.

The MR signals picked up by the multi-channel RF coil 56 are digitizedby the transceiver module 58 and transferred to a memory module 66 inthe system control 32. A scan is complete when an array of raw k-spacedata has been acquired in the memory module 66. This raw k-space data isrearranged into separate k-space data arrays for each image to bereconstructed, and each of these is input to an array processor 68 whichoperates to Fourier transform the data into an array of image data. Thisimage data is conveyed through the serial link 34 to the computer system20 where it is stored in memory. In response to commands received fromthe operator console 12, this image data may be archived in long termstorage or it may be further processed by the image processor 22 andconveyed to the operator console 12 and presented on the display 16.

FIG. 2 shows a technique 70 for MR imaging near or around patientmetallic implants according to an embodiment of the invention. In anembodiment of the invention, technique 70 will be described with respectto 3D Fast Spin Echo (FSE) MR imaging; however, it is contemplated thattechnique 70 may also apply to 3D spin-echo sequences and other 3D MRacquisition techniques. In an embodiment of the invention, computersystem 20 may be programmed to perform technique 70. Technique 70 beginswith configuring a pulse train of an MR acquisition pulse sequence atblock 72 to be used during each MR acquisition of the MR imaging toacquire a 3D MR data set. The pulse train is configured such that noslab selection imaging gradients are used. In this manner, imagereconstruction may be simplified. The shape of pulses of the pulse trainwithin the MR acquisition pulse sequence is also configured. In anembodiment of the invention, a Gaussian pulse shape is used as the shapefor pulses of the pulse train. In another embodiment of the invention,the pulse shape used may be based on a spatial-spectral pulse shape oron the shape of a hard or square pulse.

At block 74, imaging bandwidths for the MR acquisitions are determined.An excitation pulse bandwidth for the MR acquisition pulse sequence tobe used for acquiring MR data is determined as well as a bandwidth ofutilized refocusing pulses. The utilized refocusing pulse bandwidth isdetermined to be equal to or less than the bandwidth of the excitationpulse. A receiver bandwidth for the receive coil array used to acquireMR data during the MR acquisition pulse sequence is set to a bandwidthlarger than that typically used in 3D FSE imaging. In an example, thereceiver bandwidth is set to +/−125 kHz. It is contemplated that thereceiver bandwidth may also be set to a value greater than +/−125 kHz.In the described technique, off-resonance readout distortion is limitedto frequency offsets contained in the RF refocusing band. Setting thereceiver bandwidth accordingly helps to minimize this residual readoutdistortion in reconstructed images.

A resonance interval is determined at block 76 that represents an offsetfor both the center resonance frequency for transmission and the centerresonance frequency for reception between sets of acquired MR data.According to an embodiment of the invention, the resonance interval isless than the bandwidth of the utilized refocusing pulses. At block 78,a resonance interval sequence is determined for acquiring 3D MR datasets. The resonance interval sequence includes offset frequency values,or B₀ values, to which central transmission and central receptionresonance frequencies are set during MR acquisition. In an embodiment ofthe invention, the resonance interval sequence includes an offsetfrequency value of zero. Additional values in the resonance intervalsequence include multiples of the resonance interval. For example, theresonance interval sequence may include values for the centraltransmission and central reception resonance frequencies to be set toeach 1 kHz offset step in the range −7 kHz to +7 kHz.

In an embodiment of the invention, the resonance interval sequence isset to interleave or interlace the offset frequency values such thatsequential MR acquisitions based on the offset frequency values do notacquire MR data with the central transmission and central receptionresonance frequencies set to sequential offset frequency values. Forexample, an interleaved resonance interval sequence with a 1 kHzresonance interval (or offset step) in the range −7 kHz to +7 kHz mayhave the following order: [−7, 1, −5, 3, −3, 5, −1, 7, −6, 0, −4, 6, −2,4, 2 kHz]. Accordingly, neighboring values in the resonance intervalsequence are separated by more than the offset step of 1 kHz.Interleaving the resonance interval sequence in this manner reducesinteraction between 3D MR data acquisitions in an imaging scan. Asdescribed further below, each offset frequency value in the resonanceinterval sequence is used as the central transmission and receptionfrequency for a different 3D MR data acquisition. In one embodiment, anMR imaging scan (or protocol) may be configured such that a first set ofacquisitions uses a resonance interval sequence with the offsetfrequency values [−7, 1, −5, 3, −3, 5, −1, 7 kHz] during a single scanand such that a second set of acquisitions uses a resonance intervalsequence with the offset frequency values [−6, 0, −4, 6, −2, 4, 2 kHz]during another single scan. The resonance interval sequence valueslisted above are illustrative only and do not limit the invention. Otherand different orders and values for the resonance interval sequencevalues are considered and are within the scope of the invention.

At block 80, the central transmission and central reception resonancefrequencies for a 3D MR data acquisition are both set to one of thevalues in the resonance interval sequence, in particular, the centraltransmission frequency and the central reception frequency for theacquisition are set to the same offset frequency value. 3D MR data isacquired at block 82 using the scan parameters and sequences configuredand determined in the previous steps of technique 70. In an embodimentof the invention, the 3D MR data is acquired using non-parallel imagingtechniques. The 3D MR data may be acquired via multi-channel RF coil 56of FIG. 1 or via another multi-channel receive coil. However, it iscontemplated that parallel imaging techniques such as ARC, and the likemay also be used and that multiple multi-channel receive coils may beused to acquire the 3D MR data. At block 84, it is determined if another3D MR data acquisition should be performed. If all the offset frequencyvalues in the resonance interval sequence have not been used 86, thenprocess control returns to block 80 for setting the central transmissionand central reception resonance frequencies for the next 3D MR dataacquisition to another of the offset frequency values in the resonanceinterval sequence and at block 82 3D MR data for another 3D MR data setis acquired as described above. If all the offset frequency values inthe resonance interval sequence have been used 88, an image isreconstructed at block 90 for each MR data set acquired, resulting in acollection of images. Each image is reconstructed using knownreconstruction techniques.

A final, single composite image is constructed at block 92. In anembodiment of the invention, the composite image is constructed usingthe maximum intensity projection (MIP) of each pixel from the collectionof images. A pre-determined pixel location in each image of thecollection of images is used to determine which image contains thegreatest intensity projection for the pre-determined pixel location. Thegreatest intensity projection value is then used for the same locationin the final composite image.

In another embodiment, the final, single composite image may beconstructed using iterative reconstruction based on a magnetic fieldmap. FIG. 3 shows a technique 106 for constructing a magnetic field mapaccording to an embodiment of the invention. At block 108, a pluralityof reconstructed MR images are selected. In one embodiment, theplurality of reconstructed MR images are retrieved from an image storagelocation such as memory 26 shown in FIG. 1 or another computer readablestorage medium. The plurality of reconstructed images may be generatedusing the technique described above with respect to FIG. 2. In anotherembodiment, the plurality of reconstructed MR images may be generatedon-the-fly. For example, the plurality of MR images may be generatedusing the technique described above with respect to FIG. 2. At block110, the pixels for each of the plurality of MR images are examined todetermine, for each pixel location, which image of the plurality of MRimages has the maximum intensity. At block 112, each pixel location inthe magnetic field map is assigned the offset frequency value, or B₀value, to which the central transmission and central reception resonancefrequencies are set to in the image determined to have the maximumintensity for the corresponding pixel location. For example, for a givenmagnetic field map pixel location, the image acquired with the centraltransmission and central reception resonance frequencies set to 3 kHzmay have the maximum intensity for the corresponding pixel location.Accordingly, the value of 3 kHz is used for the given magnetic field mappixel location. For an adjacent magnetic field map pixel location, itmay be determined that the image acquired with the central transmissionand central reception resonance frequencies set to −4 kHz may have themaximum intensity for the corresponding pixel location. Accordingly, thevalue of −4 kHz is used for the adjacent magnetic field map pixellocation.

Returning to FIG. 2, in another embodiment, the composite image may beconstructed using a sum-of-squares method where the MR data sets areacquired using Gaussian-shaped pulses. One skilled in the art willrecognize that the steps of technique 70 may be performed in anotherorder than that described and that such is within the scope ofembodiments of the invention.

Embodiments of the invention allow for improved MR imaging near oraround metallic implants such that artifacts and image distortion arereduced. Further, embodiments of the invention are applicable in MRimaging where significant heterogeneity of the B₀ magnetic field exists.

A technical contribution for the disclosed method and apparatus is thatis provides for a computer implemented method for MR imaging ininhomogeneous magnetic fields.

In accordance with an embodiment of the invention, a magnetic resonanceimaging (MRI) apparatus includes an MRI system having a plurality ofgradient coils positioned about a bore of a magnet and an RF transceiversystem and an RF switch controlled by a pulse module to transmit RFsignals to an RF coil assembly to acquire MR images. The MRI apparatusalso includes a computer programmed to acquire a plurality ofthree-dimensional (3D) MR data sets, each 3D MR data set acquired usinga central transmit frequency and a central receive frequency set to anoffset frequency value that is distinct for each 3D MR data set. Thecomputer is also programmed to generate a single, composite image basedon the plurality of 3D MR data sets.

In accordance with another embodiment of the invention, a method ofmagnetic resonance (MR) imaging includes determining a distinct centralfrequency for each of a plurality of 3D MR data acquisitions andperforming the plurality of 3D MR data acquisitions, each 3D MR dataacquisition having a central transmit frequency and a central receivefrequency set to the distinct central frequency determined therefor. Themethod also includes generating a composite image from the plurality of3D MR data acquisitions.

In accordance with yet another embodiment of the invention, a computerreadable storage medium having stored thereon a computer programincludes instructions which when executed by a computer cause thecomputer to set a center transmission frequency and a center receptionfrequency of a first 3D MR acquisition equal to a first center frequencyoffset and to execute the first 3D MR acquisition to acquire a first setof 3D MR data. The instructions also cause the computer to set a centertransmission frequency and a center reception frequency of a second 3DMR acquisition equal to a second center frequency offset different thanthe first center frequency offset and to execute the second 3D MRacquisition to acquire a second set of 3D MR data. The instructionsfurther cause the computer to reconstruct a composite image based on thefirst and second sets of 3D MR data.

The present invention has been described in terms of the preferredembodiment, and it is recognized that equivalents, alternatives, andmodifications, aside from those expressly stated, are possible andwithin the scope of the appending claims.

1. A magnetic resonance (MRI) apparatus comprising: an MRI system havinga plurality of gradient coils positioned about a bore of a magnet, andan RF transceiver system and an RF switch controlled by a pulse moduleto transmit RF signals to an RF coil assembly to acquire MR images; anda computer programmed to: acquire a plurality of three-dimensional (3D)MR data sets, each 3D MR data set acquired using a central transmitfrequency and a central receive frequency set to an offset frequencyvalue that is distinct for each 3D MR data set; and generate a compositeimage based on the plurality of 3D MR data sets.
 2. The MRI apparatus ofclaim 1 wherein the computer is further programmed to acquire each ofthe plurality of 3D MR data sets without slab selection via imaginggradients.
 3. The MRI apparatus of claim 2 wherein the computer isfurther programmed to acquire each of the plurality of 3D MR data setsusing a receiver bandwidth of at least +/−125 kHz.
 4. The MRI apparatusof claim 1 wherein the computer is further programmed to acquire each ofthe plurality of 3D MR data sets using at least one multi-channelreception coil.
 5. The MRI apparatus of claim 1 wherein the computer isfurther programmed to reconstruct an image for each of the plurality of3D MR data sets.
 6. The MRI apparatus of claim 5 wherein the computer isfurther programmed to perform a maximum intensity projection techniqueon the reconstructed images to generate the composite image.
 7. The MRIapparatus of claim 5, wherein the computer is further programmed toperform a sum-of-squares technique on the reconstructed images togenerate the composite image.
 8. The MRI apparatus of claim 5 whereinthe computer is further programmed to generate a magnetic field map fromthe reconstructed images.
 9. The MRI apparatus of claim 8 wherein thecomputer is further programmed to perform iterative reconstruction usingthe magnetic field map to generate the composite image.
 10. The MRIapparatus of claim 1 wherein the computer is further programmed tointerleave offset frequency values used to acquire the plurality of 3DMR data sets such that sequential 3D MR data sets are acquired usingnon-sequential offset frequency values.
 11. A method of magneticresonance (MR) imaging comprising: determining a distinct centralfrequency for each of a plurality of 3D MR data acquisitions; performingthe plurality of 3D MR data acquisitions, each 3D MR data acquisitionhaving a central transmit frequency and a central receive frequency setto the distinct central frequency determined therefor; and generating acomposite image from the plurality of 3D MR data acquisitions.
 12. Themethod of claim 11 wherein performing the plurality of 3D MR dataacquisitions comprises generating a pulse train comprising one ofspectral-spatial pulses, Gaussian pulses, and square pulses.
 13. Themethod of claim 12 wherein the pulse train is configured to avoid slabselection using imaging gradients.
 14. The method of claim 11 furthercomprising reconstructing an image for each of the plurality of 3D MRdata acquisitions; and wherein generating the composite image comprisesapplying a maximum intensity projection technique on the reconstructedimages.
 15. The method of claim 11 further comprising reconstructing animage for each of the plurality of 3D MR data acquisitions; and whereingenerating the composite image comprises applying a sum-of-squarestechnique on the reconstructed images.
 16. The method of claim 11further comprising reconstructing an image for each of the plurality of3D MR data acquisitions; generating a magnetic field map from thereconstructed images; and wherein generating the composite imagecomprises performing iterative reconstruction using the magnetic fieldmap.
 17. A computer readable storage medium having stored thereon acomputer program comprising instructions which when executed by acomputer cause the computer to: set a center transmission frequency anda center reception frequency of a first 3D MR acquisition equal to afirst center frequency offset; execute the first 3D MR acquisition toacquire a first set of 3D MR data; set a center transmission frequencyand a center reception frequency of a second 3D MR acquisition equal toa second center frequency offset different than the first centerfrequency offset; execute the second 3D MR acquisition to acquire asecond set of 3D MR data; reconstruct a composite image based on thefirst and second sets of 3D MR data.
 18. The computer readable storagemedium of claim 17 wherein the instructions that cause the computer toreconstruct the composite image cause the computer to: compare relatedpixels between the first and second sets of 3D MR data; and choose oneof the related pixels having the greatest intensity for the compositeimage.
 19. The computer readable storage medium of claim 17 wherein theinstructions that cause the computer to execute the first and second 3DMR acquisitions cause the computer to execute the first and second 3D MRacquisitions without slab selection using imaging gradients.
 20. Thecomputer readable storage medium of claim 17 wherein the instructionsthat cause the computer to reconstruct the composite image cause thecomputer to: construct a magnetic field map based on at least the firstand second sets of 3D MR data; and iteratively reconstruct the compositeimage based on at least the magnetic field map and the first and secondsets of 3D MR data.
 21. The computer readable storage medium of claim 17wherein the instructions further cause the computer to: set a centertransmission frequency and a center reception frequency of a third 3D MRacquisition equal to a third center frequency offset; execute the third3D MR acquisition to acquire a third set of 3D MR data; and interleavethe center transmission and reception frequencies of the first, second,and third 3D MR acquisitions such that the center transmission andreception frequencies of one of the first and third 3D MR acquisitionsis between the center transmission and reception frequencies of theother 3D MR acquisitions.